There is an increasing public awareness of the diagnostic significance of levels of components in the blood and other body fluids, and many sensors are designed for non-expert use. A particular goal is the provision of electrochemical sensors which do not require elaborate preparation of a test sample, and can rapidly give a result merely by application of the body fluid to a test strip. Good sensitivity is achieved usually with a biochemically specific reaction, such as by enzymatic catalysis of a reaction of the component to be detected. The reaction results in transfer of charge which can be qualitatively or quantitatively detected, for instance by an amperometric procedure.
Amperometric biosensors for performing diagnostic tests for components of body fluids are described, for example, in U.S. Pat. No. 4,545,382, European Patent 127958 and European Patent 351891. Such devices offer the facility for rapid, convenient and specific measurements of analyte components in blood by lay users, and a sensor for glucose is commercially available as the MediSense.RTM. ExacTech.RTM. sensor.
Whole blood is a complex, predominately aqueous mixture containing dissolved gases, simple dissociated and hydrated ions, materials in colloidal or other form of solution, small scale cellular debris, and living cellular components including red blood cells, the erythrocytes. It is sometimes difficult to achieve reproducible analytical results with electrochemical sensors when using whole blood samples. Variations in red cell concentration, the haematocrit, is the main contributor to inaccuracies in using whole blood samples for electrochemical sensors.
Accordingly, there is a need to develop improved electrochemical sensors which are less dependent on the haematocrit of the blood. To this end, there have been proposals for overlay membranes which serve to exclude erythrocytes so that the sensor itself is only contacted by blood plasma. Illustrative examples of proposals for erythrocyte exclusion membranes are to be found in published EP-A 289269 and other patent literature.
There are difficulties in developing a satisfactory erythrocyte exclusion membrane suited for test strips such as amperometric electrode test strips. A human erythrocyte is deformable in vivo to pass through luminal diameters as small as 2 or 3 .mu.m, and thus membranes of this porosity are ineffective, either because the erythrocytes leak through by deformation or because the erythrocytes plug the pores to prevent passage of the plasma. Where the porosity is further reduced, the transport of plasma is difficult. In this respect, the typical volume of the blood sample for an amperometric glucose test is only 20 .mu.l and yet gravity has to provide the principal driving force, with capillary forces providing only a secondary driving force. Given the goal of maintaining or reducing measurement times, where the period for a complete glucose test is typically 30 seconds, rapid transport of the plasma through the membrane is needed.
Thus, the transport of the plasma requires an open membrane structure, while the separation of erythrocytes demands a closed membrane structure. In the light of these contradictory requirements, it becomes clear that conventional membrane systems are not entirely suitable.
U.S. Pat. No. 5,055,195 describes the use of retention substrates which bring about strong coagulation of blood so that the corpuscular components are effectively retained in a paper or glass fibre fleece and are separated from the plasma. This object is achieved by using retention substrates which contain two strongly polar groups which are connected by a non-polar bridge. The retention substrates are preponderantly dyestuffs, and they bring about random clumping.